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[aprssig] Packet test

scott at opentrac.org scott at opentrac.org
Mon Nov 27 16:41:39 UTC 2006


Destination address should start with AP.  You can get a product identifier
from Bob if you don't want to use the generic APRS tocall.  SSID value
should be whatever the user selects - the use of SSIDs for station type is
deprecated.  You can hard-code the control byte to 0x03 and the PID to 0xf0.
I'd have to check on the address field C bits.  I'm pretty sure I've got the
one on the tocall set to 1, and on the fromcall set to 0.

For the proper CRC-CCITT algorithm, check my page at
<http://n1vg.net/packet>.  It lists the bitwise version of the algorithm,
and has a link to the reference I used for the OpenTracker.  The bitwise
algorithm doesn't require any lookup tables and can be done with shifts and
XORs - it may not be as fast, but it's very compact.

What are you using for the AFSK modulation?  Are you doing it in hardware or
software?

Until I saw your email address, I didn't realize that you're in Santa
Barbara - I'm an hour up the road from you in Santa Maria.  Do you know
Bruce Gordon, N6OLT?  I've used his DF equipment for years.

If you want a lower traffic list for a peer review of your message, you
might try the APRSSPEC list.  It's mostly just us developers over there, and
no one minds esoteric discussions of protocol details.

Scott
N1VG

> -----Original Message-----
> From: aprssig-bounces at lists.tapr.org 
> [mailto:aprssig-bounces at lists.tapr.org] On Behalf Of R. Simmons
> Sent: Monday, November 27, 2006 7:20 AM
> To: post_message APRS
> Subject: [aprssig] Packet test
> 
> To all :
> 
>  I am developing PIC code to send an APRS DF message, ( for a 
> DF beacon )
> and I approaching completion... getting close to an "air test" of the
> software but still a few "loose ends" to clear up...
> 
>  Questions about a suitable destination address, the proper 
> SSID values and
> C-bit values. Also, the AX.25 spec says the CRC bytes should 
> be sent MSB
> first, but public code I have seen shows LSB is sent first, 
> things like
> that...
> 
>  Any ( authoritative ) help would be appreciated, and a peer 
> review of my
> intended message would also be appreciated. I can be 
> contacted "off-forum"
> at this address : pelican2 at silcom.com
> 
>  Thanks...
> 
> DE WB6EYV / Bob S.
> 
> 
> 
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> 
> 







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